Voltage drop on the AC line? Or the speaker output terminals.
Per below. Is what your saying that you don't try and measure noise on the power line that would require a spectrograph to see. You use a tube amp that is susceptible to high frequency EMF type noise and see if the amplifier is registering more noise on its output. If it is, you can reasonably assume it is from something on the line. And if less, that something was removed from the line.
Then you measure the 'before' and 'after' power, distortion and output impedance of the amp used as the load (since amps usually draw the most power). The thing about this is the amp must be at full power and one that draws a bit of power at that. It should also be an amp where the line Voltage can be shown to really affect the amp, which is a thing you can sort out using a variac with the proposed amp prior to the actual testing.
